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United States Department of Agriculture

Agricultural Research Service

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Location: Plant Genetic Resources Research

2012 Annual Report

1a. Objectives (from AD-416):
Screen at least 200 accessions for resistance to target spot (caused by Corynespora cassiicola) from the core collection at the Tomato Genetics Resource Center in Davis, CA, and at least 150 accessions from the USDA-ARS, Plant Genetic Resources Unit, Geneva, NY.

1b. Approach (from AD-416):
Because the effect of other common foliar pathogens could compromise efforts, initial screening will be performed in greenhouse trials. The inoculation protocol will be as follows. Six-eight week old plants will be sprayed with 5 x 104 conidia per ml of a 0.01% (v/v) Tween 20 solution prepared from one week old fungal cultures grown on half-strength potato dextrose agar incubated at room temperature under continuous light. Following inoculation, plants will be incubated in a mist chamber at 24- 28°C for 24 hours, and then returned to the greenhouse. Disease severity will be scored as the mean lesion diameter and number of lesions per leaf area 3 to 4 days after inoculation. All tests will be performed with at least 8 plants per accession and arranged in a completely randomized design prior to inoculation. Accessions with fewer or smaller lesions per leaf area will initially be scored as partially resistant, while those accessions with no lesions or lesions that are restricted to a degree that inhibits pathogen sporulation will be scored as resistant. All accessions identified as resistant in initial screens will be retested in subsequent screens. All screens will include the susceptible cultivar Bonnie Best and either PI 120265 or PI 112215 (resistant checks). The top resistant accessions will also be tested further in a field trial where transplants of each resistant accession, the resistant lines PI 120265 & PI 112215, and the susceptible checks FL 47 & Bonnie Best will be planted into a replicated field trial of 3 to 4 plants per a plot in a randomized complete block design with 2 to 4 blocks per an accession (depending on seed availability). Plots will be established along 300 ft beds with 5 ft center-to-center bed spacing. The tomato seedlings will be transplanted at 18” spacing along beds. Beds will be prepared and maintained based on UF-IFAS recommendations. Three to four weeks after transplanting, plots will be inoculated with a conidial suspension of C. cassiicola, prepared as described above, using a backpack sprayer. Target spot severity will be rated every two weeks as the proportion of canopy affected by disease using the Horsfall-Barratt scale. Disease severity data from field trials will be converted to mid-percentages prior to calculating area under disease progress curve (AUDPC) values using the trapezoidal method. Statistical analyses of AUDPC, lesions per leaf area, and lesion diameter will be performed using the PROC MIXED function of SAS, while the assessment of disease severity over time will utilize the REPEATED MEASURES function in PROC MIXED. Initial screening of accessions will begin in April through August 2010. Secondary screening of resistant candidates will begin in September and continue through December. The field trial will begin in February and end in June 2011. Preparation of results, including submission to GRIN database, will occur in February and March of 2011.

3. Progress Report:
For the first year of evaluations, a single isolate of Corynespora cassiicola collected from a tomato trial at the GCREC in 2008 was used to evaluate 155 PI lines and land accessions for resistance to target spot in greenhouse trials. Disease severity was scored using a 5 point scale of 0 to 4, where 0 = no symptoms; 1 = minor flecking of less than 10% of foliage or stems affected; 2 = 10 – 25% of foliage or stems affected with some coalescing of lesions and minor blighting; 3 = 25 – 50% of foliage or stems affected with coalescing lesions and moderate blighting; and 4 = greater than 50% of foliage and stems affected with severe blighting. Tests included PI 120265 a source of resistance identified by Bliss et al. (1973) and susceptible lines FL47 and SecuriTY28, which exhibited mean severity ratings of 2, 3, and 4 respectively. Of the 155 PI lines and land accessions screened, 20 were chosen that exhibited mean severity ratings less than PI 120265; of which 8 exhibited mean severity scores of 1.5 or less. All but one of the resistant TGRC land accessions identified were self-incompatible, out-crossing Solanum species that have required sib-matings to increase seed. We are in the process of increasing seed of these lines and have also requested additional seed from the TGRC (UC-Davis) to rescreen lines and accessions in subsequent greenhouse and field tests. A preliminary comparison of 15 C. cassiicola isolates collected from tomato production fields in Florida suggested that the pathogen population is quite diverse. Therefore, it will be essential to retest resistant candidates with several representative C. cassiicola isolates to identify any potential tomato genotype x C. cassiicola isolate interactions.

4. Accomplishments

Last Modified: 06/23/2017
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